The present invention relates to a cathode ray tube, and, specifically, the invention relates to an improvement in a metal film which is formed over a phosphor layer in the fluorescent screen of the cathode ray tube.
In a cathode ray tube, electron beams from electron guns are scanned on a fluorescent screen, which constitutes a display surface, to form an image on the fluorescent screen, according to the intensity of the electron beams. An evacuated envelope of a cathode ray tube typically comprises a panel portion where the fluorescent screen is formed of a number of phosphor, a neck portion housing the electron guns, and a funnel portion smoothly connecting the panel portion and the neck portion. A deflection yoke for deflecting the electron beams is arranged around the funnel portion.
The fluorescent screen formed on the inner side of the panel portion has a metal film coated thereover so as to cover the fluorescent screen. This metal film improves the brightness of the displayed image by the action of reflection and is supplied with a voltage to attract three electron beams. Normally, the metal film is called a metal reflection film or a metal back film. The metal film is formed by evaporation over an organic film resin which has been applied to the entire fluorescent screen, and then the organic film resin is removed by baking, so that the metal film remains, covering the fluorescent screen. Normally, aluminum is used for the metal film. The organic film resin is used to prevent molecules of the metal film from diffusing into the phosphor layer during the formation of the metal film, as would occur if the metal film was formed directly over the fluorescent screen.
With a cathode ray tube of the above-described construction, however, it has been pointed out that the luminance of displayed images is reduced because (1) the metal film is largely fractured, (2) the metal film as formed floats over the fluorescent screen, and (3) the metal film is not formed over a large enough area to fully cover the fluorescent screen. Our investigation into the cause of this trouble has resulted in a finding that, although an increase in the amount of organic film resin formed over the fluorescent screen results in the formation of a flat metal film without cracks or pin holes caused by degassing (generally referred to as cracks), a large amount of decomposed gas generated during the organic film resin baking process partially raises the metal film or breaks it over a wide area (hereinafter referred to as a "blister phenomenon").
Further, the organic film resin soaks into the phosphor layer. So, if the amount of the organic film resin is reduced, the organic film resin cannot cover the entire fluorescent screen. Although reducing the amount of the organic film resin can prevent the blister phenomenon in the metal film, it is found that this tends to degrade the state. of the formed metal film, leading to a reduced brightness.
The present invention has been directed to these circumstances and its objective is to provide a cathode ray tube having a metal film which is capable of preventing the blister phenomenon and reduced brightness.
Examples of a conventional cathode ray tube designed to prevent the blister phenomenon include Japanese Patent Laid-Open Nos. 191932/1982, 35834/1983 and 225723/1991.
Japanese Patent Laid-Open No. 191932/1982 discloses a technology that makes the metal film porous at corner portions and skirt portions to prevent blisters at these locations where there is no phosphor film. This publication also discloses that the drying temperature of the emulsion is differentiated between the fluorescent screen and the corner and skirt portions to make the metal film at the corner and skirt portions porous. This method, however, does not consider the prevention of blisters on the effective surface where the phosphor film is formed.
Japanese Patent Laid-Open No. 35834/1983 describes that, for the prevention of blisters, the organic film is formed by applying over the fluorescent screen a filming liquid which contains 10-20 wt% of acrylic emulsion, 0.05-0.3 wt% of propylene glycol alginate, 1-3 wt% of silicone and the remaining percentage of water, and by drying the filming liquid. It is also described that during this process the acrylic resin and the alginate separate like islands and that dislocations occur in the boundaries between the acrylic resin and the alginate, forming gaps. This technology, however, stops short of controlling the locations where the gaps occur.
Japanese Patent Laid-Open No. 225723/1991 describes that an inorganic pigment layer is formed which has relatively large raised and recessed portions in a light absorption matrix and that a part of the raised portions pierces the metal back film to form pin holes. However, it does not take into consideration the pin holes and cracks in the phosphor layer.